This invention relates to cardiac assistance devices and, more particularly, to devices designed to be secured to a natural heart for therapeutic or diagnostic purposes.
A major consideration in the design of cardiac assistance systems is the risk of thromboembolism or infection. These risks are heightened when the assist device includes blood-contacting surfaces. To avoid the need for direct blood contact, a class of devices known as “extra-cardiac assist devices” has been proposed. Extra-cardiac devices typically are implantable within the thoracic cavity and surround at least a portion of the patient's heart to provide therapy without directly contacting the patient's circulatory system.
A number of such implantable, extra-cardiac assist systems have been developed. These systems include active systems, which provide auxiliary pumping action to supplement or assist the blood pumping action of the natural heart, as well as passive systems, which support the heart without augmenting the natural heart's pumping action. Some cardiac assist devices are designed for short-term use (e.g., a few days) while others are intended for long term application (e.g., years). In many cases, it is also desirable that such devices be detachable.
For example, one class of active assist devices employs a heart-wrapping assembly formed of a flexible, but non-distensible, outer member with an elastic distensible inner membrane. An inflation fluid is then fed to one or more chambers defined between the non-distensible outer housing and the distensible inner membrane to effect pressure on the heart. In general, active assist devices fill and empty chamber(s) to compress the myocardium of the ventricle, and thus supplement the heart's natural pumping action.
There is considerable present interest in active cardiac assist devices that can be more easily applied to the heart. In emergency situations, the ability to quickly attach an extra-cardiac assist device to the heart can be critical to a patient's survival. Ideally, an extra-cardiac assist device should be quickly securable to the cardiac surface. It is likewise desirable for cardiac assist devices to be detachable without trauma to the heart and/or surrounding tissue.
Similar problems are encountered with passive cardiac assist devices, which have been proposed to prevent cardiac expansion beyond a predetermined volumetric limit in order to assist patients suffering from cardiac dilation or related conditions. In the absence of such constraint, the weakened heart muscle will deteriorate and lose its ability to pump blood. In passive devices, the goal is not to assist the natural heart's pumping action but rather to apply a constraining force during the heart's expansion (diastolic) phase.
Ideally, a passive device wrapped around the heart should mimic the natural resistance of the heart muscle itself to over-expansion. A healthy natural heart will exhibit a characteristic relationship between ventricular pressure and volume, such that small amounts of pressure at the beginning of diastole will initially result in a desired expansion of the ventricular volume. During activity or exercise, the ventricles must also respond to higher pressures to accommodate a greater volumetric expansion and, thereby, permit increased ventricular output. However, in certain disease states, the heart will increase in size over time beyond any normal volumetric range and then strain to pump blood. To arrest this dilation, which degrades cardiac performance, passive constraint devices have been proposed.
One problem that limits the effectiveness of passive devices is the need for such devices to maintain a close fit about the heart. Too loose a fit will degrade performance, while too tight a fit will put additional stress on the heart during diastolic expansion.
What is also needed is a passive cardiac device that can better mimic the heart's response to increases in diastolic pressure (in order to prevent further dilation) and, in particular, passive devices that can continue to function and respond to such pressures over time as the heart's function improves.
Conventional techniques and mechanisms for attachment of devices to the heart, such as sutures, glues, drawstrings and suction all have drawbacks, many of which directly contribute to the problems identified above. Sutures and drawstrings, for example, loosen over time and exacerbate the problem of maintaining a close fit of the device to the heart. Sutures can also cause trauma as a result of penetration into the myocardium. Suction can likewise cause trauma (e.g., hematomas) while glues can make it difficult to detach the device from the heart.
There exists a need for improved cardiac devices, generally, that can maintain contact with the beating heart (e.g., remain securely attached to the heart regardless of the state of pumping chambers or the heart itself—during both the diastole and systole phases.). This need for reliable attachment mechanisms applies not only to cardiac assist devices but to heart monitors, electrical sensors, pacemaker leads etc.